A thick piece of glass is a better insulator than two thin pieces of glass with air in the middle. This is because glass is a poor conductor of heat, and air is a good conductor of heat. When heat tries to flow through a thick piece of glass, it has to travel through a lot of glass, which makes it difficult for the heat to transfer. However, when heat tries to flow through two thin pieces of glass with air in the middle, the heat can easily travel through the air, which makes it easier for the heat to transfer.